In recent years, electrophotographic apparatuses, which commonly were used in offices, have been used increasingly for personal purposes, and there is a growing demand for technologies that can achieve, e.g., a small size, a high speed, high image quality, or high reliability for those apparatuses. Under such circumstances, a cleanerless process, a tandem color process, and oilless fixing are required along with better maintainability and less ozone emission. The cleanerless process allows residual toner from the transfer to be recycled for development without cleaning. The tandem color process enables high-speed output of color images. The oilless fixing can provide clear color images with high glossiness, transmittance, and offset resistance, even if no fixing oil is used to prevent offset during fixing. These functions should be performed simultaneously, and therefore improvements in the toner characteristics as well as the processes are important factors.
In a fixing process for color images of color printers, color toner should be melted and mixed to increase the transmittance. A melt failure of the toner may cause light scattering on the surface or the inside of the toner images, and the original color of the toner pigment is affected. Moreover, light does not reach the lower layer of the superimposed images, resulting in poor color reproduction. Therefore, it is essential for the toner to have a complete melting property and transmittance high enough not to reduce the original color. In particular, the need for light transmittance as an OHP sheet is increasing with an increase in opportunities to give a presentation using color data.
During the formation of color images, the toner may adhere to the surface of a fixing roller and cause offset. Therefore, a large amount of oil or the like should be applied to the fixing roller, which makes the handling or configuration of equipment more complicated. Thus, oilless fixing (no oil is used for fixing) is required to provide compact, maintenance-free, and low-cost equipment. To achieve the oilless fixing, e.g., the configuration of toner in which a release agent (wax) with a sharp melting property is added to a binder resin is being put to practical use.
However, such toner is very prone to a transfer failure or disturbance of the toner images during transfer because of its strong cohesiveness. Therefore, it is difficult to ensure the compatibility between transfer and fixing. In the case of two-component development, spent (i.e., the adhesion of a low-melting component of the toner to the surface of a carrier) is likely to occur by heat generated by mechanical collision or friction between the particles or between the particles and the developing unit. This decreases the charging ability of the carrier and interferes with a longer life of the developer.
Japanese patent No. 2801507 (Patent Document 1) discloses a carrier for positively charged toner that is obtained by introducing a fluorine-substituted alkyl group into a silicone resin of the coating layer. JP 2002-23429 A (Patent Document 2) discloses a coating carrier that includes conductive carbon and a cross-linked fluorine modified silicone resin. This coating carrier is considered to have high development ability in a high-speed process and maintain the development ability for a long time. While taking advantage of superior charging characteristics of the silicone resin, the conventional technique uses the fluorine-substituted alkyl group to obtain properties such as slidability, releasability and repellency, to increase resistance to wearing, peeling or cracking, and further to prevent spent. However, the resistance to wearing, peeling or cracking is not sufficient. Moreover, when the negatively charged toner is used, the amount of charge is too small, although the positively charged toner may have an appropriate amount of charge. Therefore, a significant amount of the reversely charged toner (positively charged toner) is generated, which leads to fog or toner scattering. Thus, the toner is not suitable for practical use.
Various configurations of the toner also have been proposed. It is well-known that the toner for electrostatic charge image development used in an electrophotographic method generally includes a resin component (binder resin), a coloring component (pigment or dye), a plasticizer, a charge control agent, and an additive, if necessary, such as a release agent. As the resin component, a natural or synthetic resin may be used alone or in combination. After the additive is pre-mixed in an appropriate ratio, the mixture is heated and kneaded by thermal melting, pulverized by an air stream collision board system, and classified as fine powders, thus producing a toner base. In this case, the toner base also may be produced by a chemical polymerization method. Then, an additive such as hydrophobic silica is added to the toner base, so that the toner is completed. The single component development typically uses the toner only, while the two-component development uses a developer including the toner and a carrier of magnetic particles.
Even with pulverization and classification of the conventional kneading and pulverizing processes, the actual particle size can be reduced to only about 8 μm in view of the economic and performance conditions. At present, various methods are considered to produce toner having a smaller particle size. In addition, a method for achieving the oilless fixing also is considered, e.g., by adding a release agent (wax) to the resin with a low softening point during melting and kneading. However, there is a limit to the amount of wax that can be added, and increasing the amount of wax can cause problems such as low flowability of the toner, transfer voids, a fusion of the toner to a photoconductive member, or spent of the toner component on the carrier.
Therefore, various ways of polymerization different from the kneading and pulverizing processes have been studied as a method for producing toner.
JP 10(1998)-198070 (Patent Document 3) discloses a process of preparing a liquid mixture by mixing at least a resin particle dispersion in which resin particles are dispersed in a surface-active agent having a polarity and a colorant particle dispersion in which colorant particles are dispersed in a surface-active agent having a polarity. The surface-active agents included in the liquid mixture have the same polarity, so that toner for electrostatic charge image development with high reliability and excellent charge and color development properties can be produced in a simple and easy manner.
JP 10(1998)-301332 (Patent Document 4) discloses that the release agent includes at least one kind of ester composed of at least one selected from higher alcohol having a carbon number of 12 to 30 and higher fatty acid having a carbon number of 12 to 30, and the resin particles include at least two kinds of resin particles with different molecular weights. This can provide toner with an excellent fixing property, color development property, transparency, and color mixing property.
As the release agent, e.g., low molecular-weight polyolefins such as polyethylene, polypropylene and polybutene, silicones, fatty acid amides such as oleamide, erucamide, amide ricinoleate and amide stearate, vegetable waxes such as carnauba wax, rice wax, candelilla wax, Japan wax and jojoba oil, animal waxes such as beeswax, mineral/petroleum waxes such as montan wax, ozocerite, ceresin, paraffin wax, microcrystalline wax and Fischer-Tropsch wax, and modified materials thereof are disclosed.
However, when the dispersibility of the release agent added is lowered, the toner images melted during fixing are prone to have a dull color. This also decreases the pigment dispersibility, and thus the color development property of the toner becomes insufficient. In the subsequent process, when resin fine particles further adhere to the surface of an aggregate, the adhesion of the resin fine particles is unstable due to low dispersibility of the release agent or the like. Moreover, the release agent that once was aggregated with the resin is liberated into an aqueous medium. Depending on the polarity or the thermal properties such as a melting point, the release agent may have a considerable effect on aggregation.
Further, a specified wax is added in a large amount to achieve the oilless fixing. Therefore, it is difficult to aggregate the wax with the resin that differs from the wax in melting point, softening point and viscoelasticity, and to fuse them together uniformly by heating. In particular, the use of a release agent having a predetermined acid value and a functional group may achieve the oilless fixing, reduce fog in the development, and improve the transfer efficiency. However, such a release agent prevents uniform mixing and aggregation of the resin particles with pigment particles in an aqueous medium during manufacture. Thus, there is a tendency to increase the presence of pigment as well as the release agent that are not aggregated but suspended in the aqueous medium.
There is a limit of several hundred nanometers to the particle size the release agent or the like can have by using a emulsifying and dispersing device such as a homogenizer. To achieve a smaller particle size and a uniform particle size distribution of the toner, the release agent itself is required to form a fine particle dispersion. However, the particle size distribution of the dispersion becomes an important factor. Even if the particles size of the release agent is made finer, coarse particles are not either mixed or aggregated with the resin dispersion and the pigment dispersion, but remain suspended and exist independently, while small particles are likely to adhere to the stirring shaft or the wall surface during melting, thus resulting in low productivity.    Patent Document 1: Japanese Patent No. 2801507    Patent Document 2: JP 2002-23429 A    Patent Document 3: JP 10(1998)-198070A    Patent Document 4: JP 10(1998)-301332 A